1. Field of the Invention
The invention relates to toroidal tape cores for fault current safety switches and somewhat more particularly to a method of producing such cores wherein a toroidal tape core wound from a 0.05 through 0.3 mm thick tape composed of a Ni-Mo-Cu-Fe alloy is subjected to various thermal treatments in a non-oxidizing atmosphere.
2. Prior Art
Fault current safety switches usually contain a sum current transformer which consists of a magnetic core with primary windings for connection to a circuit being monitored and with a secondary winding. Such secondary winding feeds the excitation winding of a release magnet, influencing a switch latch for a switch device. When an alternating current - fault current occurs in a circuit being monitored, a voltage arises in the secondary winding, to which the release magnet responds. This actuates the switch latch of the switch device and interrupts the circuit being monitored. Magnetic cores composed of a material with high saturation induction and high maximum permeability, given a trigger field strength, i.e., a relatively steep hysteresis loop, are generally employed as sum current transformers for fault current safety switches which are only to respond to alternating current-fault currents. Fault current safety switches with such magnetic cores, however, frequently do not trigger given pulsed dc fault currents, since the change of magnetic flux generated by a pulsed dc in the transformer is not sufficient to induce an adequate voltage in the secondary winding of the transformer to trigger the switch.
Given fault current safety switches which are required to respond to pulsed dc fault currents, as can occur, for example, in circuits with transistor controls, one therefore employs toroidal tape cores composed of so-called F-materials which exhibit a low remanence and a relatively high induction boost. Accordingly, the latter must be so high that a voltage induced in the secondary winding by a pulsating dc fault current flowing in a primary winding of the sum current transformer is adequate for the actuation of the release magnet. Further, a resonant capacitor can also be provided in the secondary circuit (for example see German Pat. No. 2,036,497).
A suitable material for a magnetic core of a sum current transformer for such fault current safety switches can comprise, among other materials, a Ni-Mo-Cu-Fe alloy consisting of 75 through 82 wt. % nickel, 2 through 5.5 wt. % molybdenum and 0 through 5 wt. % copper, with the remainder iron, along with minor amounts of deoxidation and processing additives, which has been subjected to special thermal treatment. In somewhat more detail, a toroidal tape core wound out of a 0.03 through 0.1 mm thick tape composed of the above-described alloy is annealed for 2 through 6 hours at a temperature between 950.degree. and 1220.degree. C., then subjected to a tempering treatment for 1 through 3 hours at a temperature between 450.degree. through 600.degree. C. for setting the state of high initial permeability, and, finally, is subjected to a 1 through 50 hour tempering at a temperature between 250.degree. through 400.degree. C. The tempering preferably occurs in a magnetic field whose field lines in the material being annealed extend at right angles to the later direction of the magnetic flux in the toroidal tape core is, a transverse magnetic field. In addition to a high induction boost, such toroidal tape cores also exhibit a high initial permeability. An induction boost, .DELTA.B, is defined as the difference between the induction, given saturation or maximum level control, for example, given a field strength of 15 mA/cm, and the remanence. The pulse permeability is defined as .mu..sub.I =.mu..sub.O .DELTA.B/.DELTA.H, wherein .mu..sub.O is the permeability of empty space and .DELTA.H signifies the field strength boost (see German Auslegeschrift No. 20 44 302; German Pat. No. 1,558,820 or Elektrotechnische Zeitschrift (Electrotechnical Publication) Vol. A-89, 1968, pages 601 through 604, for example).
Specific alloys within the above-described alloy compositions heretofore utilized in fault current safety switches were selected by means of a corresponding inclusion of nickel and copper in such a manner that the magnetostriction, .lambda..sub.111 in the [111]direction is approximately equal to 0. By means of an annealing treatment and a tempering treatment, a high initial permeability was thereby attained, with a crystal anisotropy K.sub.1 =0 and, finally, a low remanence was obtained with tempering in a transverse magnetic field. Overall, magnetic cores with low remanence, high pulse permeability and high induction boost were attained which also responded to pulsating direct currents in fault current safety switches.
However, in view of the high unit numbers of such toroidal tape cores that are required, the triple thermal treatment and, the particularly involved tempering in the transverse magnetic field is disadvantageous.
In instances in which a maximum induction boost is not absolutely necessary, under certain conditions one could conceive of simply omitting the tempering in the transverse magnetic field and accepting a higher remanence and a corresponding reduction of induction boost in exchange for less complicated processing procedures. The result would be a rounded hysteresis loop, which would no longer be entirely as flat. However, this, as our own investigations have shown, is not possible with alloys having .lambda..sub.111 =0, previously employed for fault current safety switches, since the induction boost then not only becomes smaller, but is no longer adequate with respect to its temperature constancy.
Namely, it was noted that the induction boost greatly decreases when deviations from that ambient temperature at which K.sub.1 =0 was precisely set for a respective alloy by means of the tempering treatment and at which, thus, the maximum of the induction boost also lies for the corresponding tempering temperature. If, for example, K.sub.1 =0 was set to an ambient temperature of 20.degree. C. by a tempering treatment, then the temperature constancy of induction boost, .DELTA.B results in the fact that the fault current safety switch still responds to a pulsating dc-fault current at an ambient temperature of 20.degree. C. but that, with a change of the ambient temperature either up or down from this value, a change of flux induced in the secondary winding of the sum current transformer no longer suffices in order to trigger the switch as a result of the reduction of induction boost.